1. Field of the Invention
The present invention relates to an exhaust gas purification device, and more particularly, to a device for purifying hydrocarbons in the exhaust gases of internal combustion engines of automobiles.
2. Description of the Prior Art
Catalysts in the form of pellets or monolith are used for the purification of exhaust gases of automobiles. The purification by catalyst of toxic components in the exhaust gases, particularly hydrocarbons, are substantially affected by the temperature of the exhaust gas, and therefore, even when a precious metal catalyst is used, it is generally required that the gas temperature is not to be lower than 300.degree. C. Therefore, when the exhaust gas temperature is low as in the operation just after the cold start up of the engine, hydrocarbons are hardly purified by the catalyst. However, since a large amount of hydrocarbons is exhausted from the engine just after the cold start up thereof, the emission of hydrocarbons in the operation at low exhaust gas temperature occupies a substantial ratio of the total emission of hydrocarbons in the operation of the engine. Therefore, it has been a problem to be solved that the emission of hydrocarbons in the cold start up operation of the engine is suppressed. It has been considered to decrease the emission of hydrocarbons by a control of engine operation. However, such a method is not desirable because it sacrifices the engine performance.
In order to meet with the above-mentioned problem it has been proposed by Japanese Utility Model Laid-open Publication 62-5820 and Japanese Patent Laid-open Publication 57-159908 to use an exhaust gas purification device which absorbs hydrocarbons in the exhaust gas at low temperature. In said publication 62-5820 an absorbent is used with a catalyst so that hydrocarbons exhausted with the exhaust gas at low temperature are absorbed by an absorbent, while at high exhaust gas temperature the hydrocarbons exhausted from the engine are purified, together with the hydrocarbons released from the absorbent, by the catalyst.
However, the materials conventionally proposed for use as the absorbent for exhaust gases are gamma alumina, porous glass, activated charcoal or the like which are not sufficient in the absorptivity. Further, these materials lose much of the absorptivity at high temperature, and therefore, when the exhaust gas temperature is in a range between the temperature at which the absorptivity starts to decrease and the temperature at which the purification by the catalyst starts to be available, hydrocarbons are exhausted with neither absorption by the absorbent nor purification by the catalyst. Therefore, there still remains in this method that the purification of hydrocarbons is not available as required.
In order to solve this problem it has been proposed by Japanese Patent Application 63-226070 , corresponding to U.S. Pat. No. 4,985,210 assigned to the same applicant as the present application to employ zeolite, particularly mordenite or Y type zeolite for an exhaust gas purification device, such materials having a high absorptivity for hydrocarbons up to a relatively high temperature. The device therein proposed is designed to be an absorbent trapper which contains said mordenite or Y type zeolite therein and is mounted in the exhaust passage on the upstream side of a catalytic converter for the purpose of decreasing the emission of hydrocarbons.
However, although the mordenite or Y type zeolite shows a highest absorptivity at 200.degree. C. for hydrocarbon gases such as C.sub.3 H.sub.6, its absorptivity decreases at 300.degree. C. to be lower than that at 200.degree. C., though the absorptivity is still relatively high. This means that a part of the hydrocarbons absorbed by the absorbent at 200.degree. C. would no longer be held by it at 300.degree. C. The hydrocarbons in the exhaust gas include in addition to C.sub.3 H.sub.6 paraffins, olefins and aromatics of various molecular weights which are supposed to be different in being subject to the absorptivity of absorbents, thought it is practically difficult to investigate the absorptivity to absorbents of each of a large number of hydrocarbon components included in the engine exhaust gases. However, as a result of researches conducted by the inventors of the present application with respect to the actual exhaust gases of automobiles it was confirmed that when the temperature of the gas introduced into an absorbent trapper containing mordenite or Y type zeolite as absorbent rises up to about 350.degree. C., the hydrocarbons absorbed by the absorbent at a lower temperature starts to be released from the absorbent. Therefore, the exhaust gas purification device according to Japanese Patent Application 63-226070 having the absorbent trapper provided on the upstream side of the catalytic converter is not very effective, though better than nothing, in that the hydrocarbons absorbed by the absorbent are again released before the catalytic converter provided on the downstream side thereof becomes active, thus allowing such hydrocarbons to be emitted to the atmosphere without being purified.